Abstract. We formulate large deviations principle (LDP) for diffusion pair (X ε , ξ ε ) = (X ε t , ξ ε t ), where first component has a small diffusion parameter while the second is ergodic Markovian process with fast time. More exactly, the LDP is established for (X ε , ν ε ) with ν ε (dt, dz) being an occupation type measure corresponding to ξ ε t . In some sense we obtain a combination of FreidlinWentzell's and Donsker-Varadhan's results. Our approach relies the concept of the exponential tightness and Puhalskii's theorem.
IntroductionLet ε be a small positive parameter, (X ε , ξ ε ) = (X ε t , ξ ε t ) t≥0 be a diffusion pair defined on some stochastic basis (Ω, F , F = (F t ) t≥0 , P) by Itô's equations w.r.t. independent Wiener processes W t and V t :is an ergodic process in the following sense. Let p(z) be the unique invariant density of ξ ε ,andX t is a solution of an ordinary differential equationẊ t =Ā(X t ) withĀ(x) = R A(x, z)p(z)dz subject to the same initial point x 0 . Then for any bounded continuous function h(t, z) and T > 0where r T is the uniform metric on [0, T ]. The above-mentioned ergodic property is a motivation to examine LDP for pair (X ε , ξ ε ), or more exactly for pair (X ε , ν ε ), where ν ε = ν ε (dt, dz) is an occupation measure on R + ×R, B(R + )⊗B(R) (B(R + ), B(R) are the Borel σ-algebras on R + and R respectively) corresponding to ξ ε :A choice of ν ε as the occupation measure is natural since the first ergodic property in (1.2) is nothing but1991 Mathematics Subject Classification. 60F10.